Star formation is a complex and beautiful process, but the reasons as to why the biggest stars get so massive has been a mystery to science.

Now, in research led by University of Toronto astronomers, it seems the fattest stars likely form when they are surrounded by a cluster of older stars, which force-feed them gas, causing them to beef-up to gargantuan proportions during their formative years.

Using data from the European Herschel infrared space observatory, gas swirling around the Westerhout 3 (W3) giant molecular cloud (GMC), some 6,500 light-years away, appears to show the ultimate buddy system at work. Corralled inside a cluster of older stars, young stars are in the perfect location to receive an optimized quantity of gas confined inside the stellar nursery from their older siblings.

The result? They’re getting bloated on the rich supply of gaseous matter, a supply they wouldn’t be able to capture if they were on their own.

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During star formation, the intense radiation generated by young stars creates an outward pressure, pushing surrounding gas away. There is a theoretical maximum of the mass of any given star; they form from the gravitational collapse of a surrounding gas cloud, ignite and then blast the remaining gas away with intense ultraviolet radiation, preventing them from growing any further. Scientists estimate that the most massive stars should grow to a maximum of eight times the mass of the sun. In W3, there are much bigger specimens — O-type stars are known to exist inside, young main sequence stars that can outshine the sun a million times up to 90 times more massive.

“The radiation during the birth of high-mass stars is so intense that it tends to destroy and push away the material from which they need to feed for further growth,” said Alana Rivera-Ingraham, a postdoctoral researcher at the Institut de Recherche en Astrophysique et Planétologie in Toulouse, France. She led the W3 study when she was graduate student at the University of Toronto.

But, as Rivera-Ingraham’s team discovered, deep inside W3 there’s another factor at play — older stars counteract this outward pressure, forcing gas into the stellar nursery.

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In their paper, the researchers say: “our results indicate that an active/dynamic process aiding in the accumulation, compression, and confinement of material is a critical feature of the high-mass star/cluster formation, distinguishing it from classical low-mass star formation.” This research will be published in the April edition of The Astrophysical Journal and an arXiv preprint is available online. The W3 cluster is basically acting like a stellar pressure cooker.

Observing stellar nurseries inside thick molecular clouds can be a tricky process, but Herschel is sensitive to the infrared radiation generated by cool dust and gas. This radiation can escape through the obscuring clouds that is opaque to visible light, highlighting where the gas is accumulating and forming stars.

“We can now see where stars are about to be born before it even happens, because we can detect the cold dust condensations,” said co-investigator Peter Martin, also of the University of Toronto. “Until Herschel, we could only dream of doing that.”

Image: The W3 star-forming region as seen in infrared light. Stars in W3 are not visible as Herschel is only sensitive to infrared. A concentration of gas is obvious in the top-left hand corner, a region where gas has been corralled by surrounding old stars creating a fertile environment for massive baby stars to grow. Credit: ESA/PACS/SPIRE/Alana Rivera-Ingraham and Peter Martin.